1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof, and more particularly to a display device in which thin film transistors are formed over a substrate and a manufacturing method thereof.
2. Description of the Related Art
An active-matrix-type display device includes pixels which are arranged in a matrix array. The respective pixels are sequentially selected in the columnar direction by supplying a scanning signal to signal lines (gate signal lines) each of which is used in common by the respective pixels which are arranged in the row direction and, at timing of such sequential selection, a video signal is supplied to the respective pixels arranged in the columnar direction through signal lines (drain signal lines) each of which is used in common by the respective pixels.
For this end, each pixel includes a thin film transistor for fetching the video signal from the drain signal line into the pixel (pixel electrode) in response to the supply of the scanning signal.
Further, on the same substrate on which the pixels are formed, a drive circuit which supplies the scanning signal to the gate signal lines or the video signal to the drain signal lines is formed, and the drive circuit is constituted of a circuit which includes a plurality of thin film transistors. The drive circuit also includes a CMOS thin film transistor which is formed by complimentarily connecting an n-type thin film transistor and a p-type thin film transistor, for example.
Here, the thin film transistor is formed as a so-called MIS-type transistor or a so-called MOS-type transistor having the following constitution, for example. That is, the MIS-type transistor or the MOS-type transistor is constituted such that a gate insulation film is formed so as to cover a semiconductor layer, a gate electrode is formed on an upper surface of the gate insulation film in a state that the gate electrode strides over the semiconductor layer, and a pair of electrodes (a drain electrode and a source electrode) which are arranged to face each other in an opposed manner with a region (channel region) above the gate electrode sandwiched therebetween is formed on an upper surface of the semiconductor layer.
Further, there has been known a thin film transistor which forms a high-density-impurity layer constituting a contact layer on portions of the semiconductor layer which are connected to the respective electrodes, and forms a low-density-impurity layer of the same conductive type on a channel region side.
The low-density-impurity layer is referred to as a so-called LDD (Lightly Doped Drain) layer and has an effect of alleviating the concentration of an electric field between the contact layer and the gate electrode.
A display device having such a thin film transistor is disclosed in JP-A-2004-54168 (patent document 1), for example.